1. Field of the Invention 
This invention relates generally to power steering apparatuses, and more particularly to a power steering apparatus that includes as an input the grip pressure a driver exerts on a steering wheel. 
2. Description of the Related Art 
It is known to provide a mechanism to assist the driver of a vehicle, in particular an automobile, in the steering of the wheels of the vehicle. Power-assist steering is desirable when the vehicle is moving slowly over a high-friction surface, such as concrete, because the steering wheel of such a vehicle can be especially difficult for a driver to turn under such conditions. 
At higher driving speeds and under other conditions, it is desirable to have little or no assist in order to maintain the amount of feedback provided by the steering system to the driver. Thus, variable-assist power steering systems have been developed for providing a relatively high amount of power assist when desired, and for providing a relatively low amount of power assist when desired, such as when driving at highway speeds when the steering wheel is at or near the center position. 
Conventional variable-assist power steering systems determine the amount of power assist the system should provide based upon one or more variables, including engine speed, automobile speed and wheel torque, and thereby increase or  decrease the amount of assist based upon the values of those variables. Although conventional power-assist systems provide a reasonable balance between the desire for feedback to the driver and power assist to aid in steering, the need exists for a system based upon more complete data to more accurately determine the amount of assist that is desirable under virtually all circumstances. 